1. MODEMS (Converts analog signals to digital and vice-versa)
Lecture 7
The second semester: 2017\1438
T. Elham Sunbu

2. OUTLINE: Modulator – converts digital information to analog
Demodulator – converts analog information to digital
RS-232 Interface

3. MODEMS
A modem is a device or program that enables computer to transmit data over telephone lines.
Computer information is stored digitally, whereas information transmitted over telephone lines is transmitted in the form of analog waves. A modem converts between these two forms.

4. Basic Principle
0101

6. Basic Structure

7. How Modulation is used for data transfer ?
the transmitted symbol sequence will be:

8. Classification of Modems
Range
Line Type
Operation Mode
Synchronization
Modulation
Data Rate

9. Range Line Type Dial-up Short Haul Leased Voice Grade Private
Wide Band

10. Operation Mode ModulationAM FM PM
TCM
Simplex
Half-Duplex
Full-Duplex

11. CABLE MODEM
Cable Modems make access to TV cable, and let you get  high-speed internet access.
Availability is still limited in many areas, but is growing rapidly.

12. EXTERNAL MODEM
It usually plug in through a serial port, or  a USB port. 
External modems cost the most, but have the benefits of being easily installed.
Also plugs into telephone line.
External power supply is needed.
Holder case is required.
Often they have the  indicators that tell the status of your connection there is one standard interface for connecting external modems to computers called RS-232.

13. TYPICAL INDICATERS ON EXTERNAL MODEMS FRONT PANAL
HS = HIGH SPEED
AA = AUTO ANSWER
CD =CARRIER DETECT
OH =OFF HOOK
RD =RECIVED DATA
SD =SEND DATA
TR =TERMINAL READY
MR = MODEM READY

14. INTERNAL MODEM It can insert into a vacant expansion slot.
No external power supply is needed.
No holder case is required.
Least expensive at given speed.
Incompatible with different kind of PCs.
These modems go inside the computer, and usually plug directly into the motherboard.

15. ISDN MODEM
ISDN is an abbreviation for Integrated Services Digital Network.
There is no such thing as a noisy or fuzzy line. 
always runs at 64kbps.
Ability to bond multiple "B" channels to get speeds of 128kbps.
Call Setup is very fast (dialing, handshake and authentication), usually under 2 seconds (compared with 15 seconds at least for most analog modems).
Ability to get 2 calls at the same phone number (one line is for voice, and the other line for your internet connection).

16. WIRELESS MODEM Convert digital data into radio signals and back.
Support a maximum data transmission rate of 19.2Kbps (kilobits per second).
Wireless modems can be used up to miles (30-50 KM).
Operation is in the 900 MHz and 2.4 GHz. 
Modem names include –
BreeseLINK, LATNET-Radio Data Links, AirLink Wireless Modems,
and Open Minds.

17. DSL MODEM
DSL Modems: DSL (Digital Subscriber Line)  service is usually offered through  the phone the company/ISP.
You can leave your Internet connection open and still use the phone line for voice calls.
The speed is much higher than a regular modem (up to 1.5 Mbps )
DSL doesn't necessarily require new wiring; it can use the phone line you already have

18. Modem Speed The Modulation Speed The Serial Port Speed Note:
Modem speed is rated in bps (bits per second).  So a 56k modem transmits at 56,000 bits per second in theory. While 8 bits make a byte, and 1024 bits equal one kilobits, a 56k modem will transmit a maximum of approximately 7 kilobytes of information every second.   A modem will never connect higher than what it is rated for

19. Modem Installation
Modem Cable Selection
Selection of Port

20. What is RS232 RS means Recommended Standard.
RS 232 uses both synchronous and asynchronous communication.
In microcontrollers like ATmel 89C51 asynchronous communication is used and it is called as UART – universal asynchronous receiver and transmitter
PCI Express x1 card with one RS-232 port

21. Interfacing RS232
We need a driver to use the RS232 with microcontroller
That driver turned out to be MAX232
It is a16 PIN Ic which is used to interface RS232 with any other devices.
The transmission speed in the RS232 is called as baud rate.

22. Setting serial port
In microcontroller there are some special function registers which are allocated to control the serial communication.
The list of registers which is used to control the serial port are
SCON
PCON
SBUF